1. Field of the Invention
The present invention generally relates to an electrospinning method for making single crystal MoO3 nanowires, nanowires made from said process and to bio-chem sensing probes comprising the nanowires of the invention.
2. Background of the Invention
One-dimensional metal oxides are the focus of current research efforts in nanotechnology as they promise improved electro-optical, electro-chromic, catalytic, and gas sensing properties. The large surface area to volume ratio of nanofibers and nanowires suggest improvement of adsorption and reaction rates of gas sensitive materials. Electrospinning is a novel nanomanufacturing technique used to process metal oxide nanofiber networks. Titanium dioxide was among the first metal oxides that were processed by means of electrospinning into composite nanofibers. Li and Xia, NanoLetters, (2003), 3(4), 555-560. In the method of Li and Xia, titania sol-gel was directly added to an alcohol solution containing polyvinylpyrrolidone (PVP) and was electrospun to form non-woven mats. Polycrystalline metal oxide nanofibers were achieved by a heat treatment at 500° C. in air for three hours.
Madhugiri and Sun et al. combined the methodology for forming mesoporous TiO2 with electrospinning to produce mesoporous titanium dioxide fibers Electrospun mesoporous titanium dioxide fibers. S. Madhugiri, B. Sun, P. G. Smirniotis and J. P. Ferraris: Microporous and Mesoporous Materials, (2004), 69, p. 77.
Most recently long titania nanofibers and those modified with erbium oxide were fabricated by electrospinning followed by thermal pyrolysis. V. Tomer, R. Teye-Mensah, J. C. Tokash and N. Stojilovic: Solar Energy Materials and Solar Cells, (2005), 85(4), 477-488.
Other groups have also reported the synthesis of numerous other metal oxide nanofibers prepared by electrospinning. They have successfully electrospun polyvinyl acetate (PVAc) with vanadium sol-gel to create composite nanofibers, and calcinations of the as received membranes resulted in pure vanadium pentoxide nanofibers. Viswanathamurthi and Bhattaij et al., Scripta Materialia, (2003), 49, 577-581. A similar procedure was followed for magnesium titanate, Dharamaraj and Park et al. Inorg Chem Comm. (2004), 7, 431; p-type semiconducting palladium oxide, Viswanathamurthi and Bhattarai et al., Materials Letters, (2004), 58, 3368-3372; nickel titanate, Dharmaraj and Park et al. Materials Chemistry and Physics, (2004). 87: 5-9, and ruthenium doped titanium dioxide, Viswanathamurthi and Bhattaij et al., Inorg Chem Comm, (2004), 7, p. 679.
Shao et al. have electrospun polyvinyl alcohol (PVA) mixed with various metal oxide solutions to create metal oxide composite nanofibers, followed by calcinations of precursor membranes to result in pure metal oxide nanowires. To date, their group has successfully synthesized Co3O4, NiO, CuO, Mn2O3, Mn3O4, ZnO, ZrO2, NiO/ZnO, NiCo2O4, CeO2, and LiMn2O4 nanofibers e.g., Materials Chemistry and Physics, (2003), 82, p. 1002-1006. The common characteristic for all published work on electrospun metal oxide nanofiber formation is the polycrystalline grain morphology seen along the decomposed polymer frame of the heat-treated composite fibers. The present document, however, has focuses on the breakthrough synthesis of single crystal metal oxide nanowires for gas sensing applications.
There are several reasons why single crystal nanowires perform better than their polycrystalline counterparts, as far as their gas sensing properties are concerned. The first reason is gas selectivity. Semiconducting oxides exhibit strong affinity to specific compounds as a function of their polymorphic structure or crystallographic arrangement. Since polycrystalline materials are homogeneous with respect to the type of crystal planes and directions exposed to the gas, there is limited gas selectivity and a lot of cross-interference from other gases observed. In contrast, single crystals are usually grown along preferred crystal planes and directions, so gas sensors based on them may markedly improve their gas sensing selectivity. The second reason is stability. Single crystalline nanowires are usually stoichiometrically better defined and have a greater level of crystallinity than the multigranular oxides currently used in sensors, they may potentially reduce the instability associated with defects forming at grain boundaries.
Finally, single crystalline nanowires have many advantages in many other applications. For example, light emitting devices and field emission devices based on single crystalline semiconducting oxide nanowires have much stronger signal intensities than polycrystals.